1. Field of Invention
The present invention relates to a communication system and communication method, in particular, to a communication system incorporating mobile communication network and video broadcast network as well as the communication method thereof.
2. Description of Prior Art
As the rise of the demand for mobile audio and video, mobile video broadcast service has increased in the market of mobile communication. Especially, as the wideband wireless technology developed rapidly, more and more discussion is directed to how to incorporate the conventional mobile communication network, wireless Internet data network and broadcast network in the next generation of wireless network.
The conventional mobile communication network uses a cellular structure. Such cellular structure performs communication in the manner of frequency division multiplexing (FDM) and generally supports only voice and data unicast services. FIG. 1 shows a conventional cellular architecture. As shown in FIG. 1, generally, network operators carries out networking with seven cellular architecture. That is, one cell together with six neighbor cells constitutes one cell cluster. The resultant frequency spectrum resource, for example, 14 MHz, is allocated to each of the cellular base stations respectively, which is in turn allocated with a fixed frequency band of 2 MHz. This frequency spectrum resource is repeatedly allocated among all cell clusters in a 2-dimension fashion since the cellular base stations in two neighbor cell clusters are far enough from each other to multiplex the same 2 MHz frequency band. With such cellular structure, each of the cellular base stations uses a frequency band which does not interfere with those of the other stations. Therefore, the cellular base stations in all cell clusters may operate simultaneously. Each of the cell clusters shown as in FIG. 1 contains seven cellular base stations. It will be understood that each cell cluster can also contains three or four cellular base stations. Further, any other number of cellular base stations can also be provided according to network design.
At present, however, the mobile communication network has not substantially incorporated with the video broadcast network, and the two networks are operated independently of each other. Even video content can be received over some mobile communication networks, the video stream is only treated as common data and transferred from a cellular base station to terminals for display. Therefore, the efficiency for broadcasting video programs over current cellular networks is low. Moreover, the mobile communication networks have the following problems.    1. The allocation of the frequency spectrum is fixed, so it is impossible sufficiently to use the frequency spectrum resource, thereby resulting in a waste of frequency spectrum. For example, a cellular base station 1 uses the same frequency spectrum resource as that of a cellular base station 3, whereas the number of users communicating through the base station 1 may actually smaller than the number of users communicating through the base station 3. Although the area of a cell can be dynamically increased or decreased by a method called “Cell Breathing” so as to alleviate the waste of the frequency spectrum, this method greatly complicates the overall system due to, for example, user switching and resource allocation. Further, with the method of “Cell Breathing”, it is required to change the transmitting power of the base stations to obtain a larger coverage area when the area of the relevant cell is dynamically expanded.    2. The conventional network architecture requires a complex planning for the frequency spectrum since every cell needs to be allocated with certain frequency spectrum resource.
As for video broadcast network, the conventional video broadcast transmission towers provide only analog video broadcast service. Even if a few of broadcast transmission are capable of providing digital video broadcast service, they are each networked individually without combining with any mobile communication network.